The invention relates to an extruder screw consisting of a screw shaft with several screw elements, which can be or have been detachably slipped onto this shaft, the screw shaft having an external gearing and the screw elements having an internal gearing, which engages the external gearing.
Such extruder screws, which are also referred to as plug-in screws, are known and make a variable construction of an extruder screw possible in that, depending on the requirements, different screw elements, such as conveying, kneading or mixing elements may be disposed in different sequences on the screw shaft. In order to make it possible, on the one hand, to slip on these elements and, on the other, to transfer the torque, required in operation, from the screw shaft, into which the torque is passed by way of the extruder motor, a shaft hub gearing is provided between the screw shaft and the shaft elements, that is, the screw shaft has an external gearing, whereas the screw elements, functioning as hub, have an internal gearing on the inside of their borehole, both gearings meshing with one another.
Usually, shaft hub connections, conforming to the Standards DIN 5480, DIN 5464 or ISO 4156 in the form of an evolvent gearing, are used for extruder screws. This positive, symmetrical gearing enables an appreciable torque to be transferred, while, at the same time, the screw elements are easy to install and to dismantle.
Extruders generally must meet the requirements of the highest possible efficiency, which is reflected primarily in the achievable throughput. Moreover, within the framework of machine design, the available torque, that is, the torque that can be transferred effectively over the screw shaft to the screw elements, the rpm of the screw and, with that, the drive performance, as well as the available free screw volume represent the deciding design criteria. Generally, a high available torque is an advantage, because it permits, on the one hand, a higher degree of filling. Moreover, lower average shear velocities and lower product temperatures are achieved. The residence time of the product in the extruder is shortened, the stress on the product, on the whole, is less than in comparable process steps with a lower torque. On the whole, a higher torque permits a higher screw rpm and, with that, also a higher throughput.
However, there are limits to increasing the torque, primarily due to the characteristic values of the material of construction of the screw shaft and of the screw elements and the configurational strength of the elements, which is determined by the construction of the gearing. These limits, in the final analysis, determine the maximum torque that can be transferred. Admittedly, by certain material-specific after-treatments, slight increases in torque can still be achieved. However, these are marginal and, as a rule, associated with high costs.
A further possibility for increasing the throughput is to increase the available volume, which is usually given as the volumeness in the form of the ratio of the outer diameter of the screw Da to the internal diameter of the screw Di. Usual volumenesses of double screw extruders, rotating in the same direction, range from 1.4 to a maximum of 1.6, for instance. Enlarging the volume by deepening the screw channels while keeping the distance between the axles the same admittedly offers an improved feed and less shear. At the same time, lower material temperatures are reached because of the lower input of energy, so that finally, the extruder can be run at a higher rpm, thus increasing the throughput. However, it is a disadvantage here that, as the volumeness increases and the distance between axles stays constant, the screw shaft becomes thinner or the wall of the screw element becomes very thin. At large volumenesses of more than 1.6, these geometrically induced circumstances make it impossible to transfer high torques anymore, since the forces, transferred by the screw shaft to the screw elements during the transfer of torque, lead to local, impermissible high stresses and finally to damage to the shaft-hub connection.